1. Field of the Invention
This invention relates to an electrophotographic toner.
2. Description of the Prior Art
Various electrophotographic processes have hitherto been known as described, for example, in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 23910/1967, 24748/1968, etc. In general, an electrostatic latent image is formed on a light-sensitive member by various procedures utilizing a photoconductive material and developing with a toner, and the
Procedures for developing an electrostatic latent image using toners include the magnetic brush process described in U.S. Pat. No. 2,874,063, the cascade developing process described in U.S. Pat. No. 2,618,552, the powdery mica process described in U.S. Pat. No. 2,221,776, etc. In any of these procedures, the step of fixing the toner image is, as a matter of course, essential.
In heat roll fixing equipment, the toner image comes into contact with the heat rolls in the molten state and as a result a part of the toner image attaches to the surface of the heat rolls and is transferred thereto. This phenomenon is the so-called offset phenomenon. As a result, the toners employed in a high speed copying apparatus in which the heat roll fixing equipment is used must be made such that they are free from offset.
More difficult problems are encountered in producing offset-free toners which are to be applied to a high speed copying apparatus in which heat rolls are used and not much electric power is used, i.e., so-called power saving high speed copying apparatuses.
Binder resins for use in the production of offset-free toners are required to be as tough as possible. To meet this requirement, high molecular weight polymers are usually employed. However, in order to provide these high molecular weight polymers with sufficient melt-fluidity, the toners must be heated to considerably high temperatures, which conflicts with the requirement of power-savings. Thus, in order to save power or energy, the toners should be fixable at low temperatures. In order to make the toners fluid even at low temperatures to meet the above requirement, it is desirable to employ resins having as low molecular weights as possible as well as low glass transition points. Such low molecular weight resins, however, are naturally insufficient in toughness and therefore offset is liable to take place.
Increasing the toughness of the toners in order to prevent offset has hitherto been achieved by employing high molecular weight polymers having a weight average molecular weight of more than about 100,000. As such high molecular weight polymers, vinyl based polymers are often used because vinyl based polymers having such high molecular weights can be easily produced by conventional free radical polymerization. In order to be able to fix these high molecular weight vinyl based polymer-containing toners at low temperatures, the glass transition point of the polymers has been reduced to as low levels as possible within the range that no blocking takes place, the fixing temperature has been reduced by adding a plasticizer, or other modifications have had to be made.
The above techniques, however, not only decrease the fixing point (i.e., the lowest temperature at which the fixation is completely conducted), but also decrease the hot offset temperature (i.e., the temperature at which the offset begins to take place), and thus merely shift the temperature range between the fixing point and the hot offset temperature (the so-called fusing latitude) to the low temperature side. This leads to the formation of a new problem that offset and streaks often occur due to the instability of the toners. Thus the advantage of lowering the fixing temperature is for naught. In the case that the glass transition point of the polymer is reduced or a plasticizer is added in anticipation of a decrease in the hot offset temperature, if the decrease in the fixing temperature brought about by these procedures is compensated by an increase of the weight average molecular weight, the decrease in hot offset temperature can be prevented. However, an increase in viscosity due to an increase in the molecular weight reduces the effects of the reduction in glass transition point, the addition of plasticizers, etc., and thus the fixing temperature is insufficiently lowered.
On the other hand, condensation type resins such as polyester resins and epoxy resins have low glass transition points unlike vinyl based polymers and furthermore it is possible low molecular weight condensation type resins are more easily formed. This means that the use of such condensation type resins permits easy formation of low temperature fixable toners.
With these resins, however, the offset phenomenon vigorously takes place because they are low molecular weight resins, and thus they cannot be used in toners for heat rolls. It is surprising that the elastic properties in the molten state, which are needed to prevent offset, cannot be obtained even by increasing the molecular weight of the polyester or epoxy resin. In addition, unfavorable problems occur in that only the toughness in the solid state is increased and the pulverization step, where after being molten-mixed with a colorant, the resin is pulverized to form toners, becomes quite difficult. For the above reasons the toners used with heat roll fixing equipment have not used the polyester or epoxy based resins.
On the other hand, when a mixture of toners and carriers is used as a developer, the toners alone separate from the developer mixture and participate directly in development in any of the cascade process and the magnetic brush process. In this case, the efficiency of developing an electrostatic latent image on a drum and the efficiency of transferring the so developed toner image to paper or other medium are quite important to the toners. In order to improve the developing efficiency and the transferring efficiency, charging controllers have often been used. In this case, if the charging controllers are not uniformly dispersed in the toners, the desired effect cannot be obtained or the toner quality becomes unstable. In the case of polyester or epoxy resins into which it is possible to introduce functional groups capable of acting as charging controllers, the problem of the dispersibility of the charging controllers can be avoided.
Owing to these properties, many of the toners prepared using the polyester or epoxy based resins exhibit excellent developing characteristics. As a matter of course, the introduction of functional groups having a charge controlling action into the side chains of the vinyl based polymers enables them to exhibit excellent developing characteristics like the polyester or epoxy resins.
Styrene based polymers have the defects that they have low glass transition points as compared with the polyester or epoxy resins and that it is difficult to produce low molecular weight styrene based polymers.
It is quite difficult to obtain low temperature melting materials which are equal to the polyester or epoxy resins in their developing characteristics. From the above description, it will be apparent that it has been desired to produce those toners in which the excellent low temperature fixability and the developing characteristics of the polyester or epoxy resins are present and which are free from offset to the high temperature at which tones formed from high molecular weight vinyl polymers are also free from offset.
Furthermore, it will be apparent that if it were possible to produce low temperature melting materials, which are as good as the polyester or epoxy resins, from vinyl based polymers containing functional groups having a charge controlling action, it would be desirable to produce those toners for use with heat roll equipment without losing their low temperature fixability and developing characteristics.